Conventional sedimentation or filtration systems operating under natural gravity have a limited capacity for separating a fluid/particle or fluid/fluid mixture, otherwise known as a feed slurry, having density differences between the distinct phases of the slurry. Therefore, industrial centrifuges that produce large centrifugal acceleration forces, otherwise known as G-levels, have advantages and thus are commonly used to accomplish separation of the light and heavy phases. Various designs of industrial centrifuges include, for example, the decanter, screenbowl, basket, and disc centrifuge.
Industrial centrifuges rotate at very high speeds in order to produce large centrifugal acceleration forces. Several problems arise when the feed slurry is introduced into the separation pool of the centrifuge with a linear circumferential speed less than that of the centrifuge bowl.
First, the centrifugal acceleration for separation is not fully realized. The G-level might be only a fraction of what is possible. The G-level is proportional to the square of the effective acceleration efficiency. The latter is defined as the ratio of the actual linear circumferential speed of the feed slurry entering the separation pool to the linear circumferential speed of the rotating surface of the separation pool. For example, if the acceleration efficiency is 50 percent, the G-level is only 25 percent of what might be attained and the rate of separation is correspondingly reduced.
Second, the difference in circumferential linear speed, between the slurry entering the separation pool and the slurry within the separation pool which has been fully accelerated by the rotating conveyor and bowl, leads to undesirable slippage, otherwise known as velocity difference, and this creates turbulence in the slurry lying within the separation pool. Such turbulence results in resuspension of the heavy phase, equivalent to a remixing of the heavy phase material and the lighter phase material.
Third, because a portion of the separation pool is used to accelerate the feed slurry, the useful volume of the separation pool is reduced, and thus the separation efficiency of the centrifuge is lessened.
Fourth, the feed slurry often exits the feed accelerator and enters the separation pool of the centrifuge in a non-uniform flow pattern, such as in concentrated streams or jets, which causes remixing of the light and heavy phases within the separation pool.
These problems are common in decanter centrifuges generally including a rotating screw-type conveyor mounted substantially concentrically within a rotating bowl. The conveyor usually includes a helical blade disposed on the outside surface of a conveyor hub, and a feed distributor and accelerator positioned within the conveyor hub. A feed slurry is introduced into the conveyor hub by a feed pipe, engages the feed distributor and accelerator, and then exits the conveyor hub through at least one passageway between the inside and outside surfaces of the conveyor hub. Normally the feed slurry exits through the passageway at a circumferential speed considerably less than that of the separation pool surface, thus creating the aforementioned problems. Therefore, it is desirable to incorporate feed slurry accelerator enhancements into the passageway so that the acceleration and separation efficiency of the centrifuge may be increased.
It is often desirous to wash the compacted cake solids that form on the inside surface of the bowl with a wash liquid for the purpose of either removing impurities or recovering a valuable mother liquor that may remain within the compacted cake solids. In a screenbowl centrifuge, washing of the compacted cake solids is performed on a screen section of a wash feed compartment section integral with the conveyor hub as the cake solids are conveyed along the screen section by the conveyor screw. A wash liquid is generally introduced into the wash feed compartment by at least one wash pipe. A plurality of wash nozzles extending radially from the wash compartment and proximate to the cake delivers the wash liquid to the cake. In a pusher-type centrifuge, washing of the compacted cake solids is performed on the basket of the centrifuge as the cake solids are conveyed along the basket by the pushing mechanism. A wash liquid is generally introduced into the pusher-type centrifuge by a pump, wash pipe and a plurality of nozzles. The wash liquid is disposed onto the cake surface in the form of a pressurized liquid stream.
When a wash liquid nozzle is positioned too close to the cake surface, the opening of the nozzle often becomes plugged with solids. In addition, the wash liquid channels through the cake resulting in only a small portion of the cake solids being washed.
To avoid such problems, the wash nozzle is positioned at a distance farther from the surface of the cake. In the case of a screenbowl centrifuge, the wash liquid is introduced onto the cake from the rotating wash feed compartment via nozzles at a smaller radius and will not achieve approximately the same circumferential velocity of the cake which is located at a larger radius. Several problems result when the wash liquid is not accelerated to the circumferential velocity of the cake. For example, the underaccelerated wash liquid slips relative to the rotating cake surface. Moreover, the wash liquid does not have the adequate centrifugal force to penetrate the cake, and thus, runs off the surface of the cake resulting in a poor and an uneven wash of the cake solids.
When a wash nozzle used in a pusher-type centrifuge is positioned at a distance from the surface of the cake, the pressurized wash liquid is brought to the circumferential velocity of the cake solids by adjusting the flow rate of the wash liquid for a given nozzle size. Consequently, other wash rates can not be easily accommodated without changing the wash nozzle dimensions. In this case, it is preferrable to introduce the wash liquid by means of a rotating wash feed compartment section including at least one multispray nozzle as more fully described below.
To achieve a desirable wash of the cake solids and a reliable washing operation, the wash liquid must be adequately and uniformly distributed onto the surface of the cake, the linear circumferential velocity of the wash liquid must be approximately equal to the circumferential velocity of the cake on the screen section of a decanter centrifuge or the basket of a pusher-type centrifuge, and the wash liquid nozzle or nozzles must be at a radial distance from the cake surface to prevent the openings of the nozzles from plugging.